1. Field of the Invention
The present invention relates to an X-ray spectrometer and also to a sample analyzer.
2. Description of Related Art
Known X-ray spectrometers include energy-dispersive X-ray spectrometers (EDS) and wavelength-dispersive X-ray spectrometers (WDS).
In an energy-dispersive X-ray spectrometer, X-rays produced from a sample are directly detected by a semiconductor detector and converted into an electrical signal for spectroscopic analysis.
For example, in an X-ray fluorescence (XRF) analyzer equipped with an energy-dispersive X-ray detector and disclosed in JP-A-2007-327902, X-rays are first detected by a semiconductor detector. The output from the semiconductor detector is amplified by a preamplifier and a pulsed voltage signal of staircase waveform is produced. The pulsed voltage signal is shaped into pulses having pulse heights corresponding to the heights of the steps of the staircase waveform. The pulses are digitized by an A/D converter and discriminated by a multichannel analyzer according to their pulse heights. The numbers of pulses in different pulse height ranges are counted, and a graph of a distribution of pulse heights (energy spectrum/histogram) is created.
FIG. 9 is a functional block diagram of a related art X-ray spectrometer 1, showing its configuration.
In the X-ray spectrometer 1, the output signal from an X-ray detector 10 is analyzed by signal processing circuitry 2 by a pulse-height technique. The result of the analysis is obtained by a personal computer (PC) 50 and displayed as a spectrum.
Processing performed by the signal processing circuitry 2 is now described by referring to FIGS. 10A-10B, 11A-11C, and 12. FIGS. 10A and 10B depict the processing performed by a main filter 20. FIGS. 11A-11C depict the processing performed by an event detection and processing portion 30. FIG. 12 depicts the processing performed by a pulse height analysis and processing portion 40.
The main filter 20 converts the output of a staircase waveform S10 from the X-ray detector 10 shown in FIG. 10A into a pulsed signal S20 shown in FIG. 10B, where the vertical axis indicates X-ray energy level.
The event detection and processing portion 30 converts the output of staircase waveform S10 from the X-ray detector 10 shown in FIG. 10A into a pulsed signal S3 as shown FIG. 11B, where the vertical axis indicates X-ray energy level. The event detection and processing portion 30 sets a threshold value TH for the pulsed signal S3. As shown FIGS. 11B and 11C, when the pulsed signal S3 exceeds the threshold value TH, the event detection and processing portion 30 outputs an event signal S30.
The pulse height analysis and processing portion 40 analyzes the peak heights of the pulsed signal S20, based on the output signal S20 from the main filter 20 and on the event signal S30 from the event detection and processing portion 30. As shown in FIG. 12, the pulse height analysis and processing portion 40 starts an operation for detecting a maximum value of the pulsed signal S20 in response to the event signal S30. Then, the pulse height analysis and processing portion 40 detects a maximum value of the pulsed signal S20 within a period L corresponding to the time constant of the main filter 20. The result is sent as an X-ray energy signal S40 to the personal computer 50.
The personal computer 50 discriminates the pulses of the X-ray energy signal S40 according to X-ray energy level, counts the numbers of the pulses in the individual energy levels, and converts them into an X-ray spectrum in which the vertical axis indicates the number of counts and the horizontal axis indicates X-ray energy level.
In the X-ray spectrometer 1, the event detection and processing portion 30 regards every X-ray photon exceeding the threshold value TH as an X-ray event and produces the event signal S30. However, as shown in FIG. 13, the pulsed signal S3 may involve noise. This noise may exceed the threshold value TH.
Since the event exceeding the threshold value TH is a noise, this event has an energy level of 0. In spite of an event with 0 energy level, the event signal S30 is output. Therefore, the pulse height analysis and processing portion 40 performs an operation for detecting a maximum value. Accordingly, in the personal computer 50, the event is counted as if having a low energy level rather than energy level 0. As a result, the background intensity on the lower energy side of the X-ray spectrum may be increased or the X-ray spectrum may be observed to have a peak corresponding to an element not present in the sample.
FIG. 14 is an X-ray spectrum generated when B (boron) is measured by the related art X-ray spectrometer 1.
As shown in FIG. 14, a peak is observed on the lower energy side of the X-ray spectrum in spite of the fact that no element is present in practice.